Gas-phase polymerization has been recognized as one of the most economical methods of manufacturing various polyolefin products. Major polyolefin products include polyethylene and polypropylene. Processes of manufacturing polyolefin products include the UNIPOL™ Polyethylene Process of Univation Technologies LLP, and the UNIPOL™ Polypropylene Process of the Union Carbide Corporation, which is a wholly owned subsidiary of the Dow Chemical Company. In gas-phase processes generally, a high-activity catalyst is usually fed into a fluidized-bed reactor in the form of very small particles, either in the form of dry powder catalyst, or in the form of slurry catalyst, e.g., solid catalyst in liquid medium such as mineral oil. In the reactor, monomer and co-monomer(s) are converted to polymer that grows on the catalyst particles. When the polymer product particles are discharged from the reactor, the quantity of the catalyst in the product is so small that no catalyst separation is needed. Because of the nature of such gas-phase processes, sizes of the catalyst particles are often kept small, which places the catalyst particles in the Group C powder of the Geldart's particle classification (Powder Technol. Vol. 7, p. 285, (1973)), e.g., between a few to about 200 microns.
A problem associated with a Geldart's Group C powder is the cohesiveness of the powder resulting from strong inter-particle forces, such as electrostatic forces, van der Waals forces, and the like. Accordingly, achieving a uniform distribution of the catalyst in a gas-phase reactor may be problematic. If the distribution of fresh catalyst particles in the reactor is not uniform, some local catalyst-rich spots in the reactor may be formed. As a result, excessive polymerization heat can be generated at those spots, resulting in over-heating and the melting of the polymer at those locations. When the problem is severe, large-size polymer agglomerates and even polymer “chunks” and/or “sheets” can be formed, potentially blocking the product-discharge port. As a result, the reactor may have to be shut down for cleaning, which increases costs due to loss of reactor production time, cleaning costs, and startup costs.
Further, when fed in the form of dry powder catalyst, cohesion of the catalyst may complicate maintaining a stable solid-catalyst flow rate (or feeding rate) and accurately measuring the catalyst flow-rate. That can further affect the stable operation of the reactor. In addition, maintaining a relatively high flow rate of a cohesive dry catalyst may require multiple catalyst feeders in large size reactors, which increases the investment and operational cost of the reactor system.
Commercially, different methods have been adopted to fight the problem of cohesive catalyst powder. One common method is to make a slurry of catalyst with an inert liquid, such as mineral oil or a paraffinic solvent, so the accurate feeding measurement and control would be less of a problem. However, the catalyst distribution inside a reactor may still be uneven with the slurry catalyst, because the inert liquid might be quickly separated from the catalyst particles after feeding into the reactor, via contacting with numerous vigorously moving particles inside the reactor. The slurry catalyst feed has its own advantages and disadvantages, such as requiring additional slurry-making equipment and procedure, impact on catalyst kinetics, and the like. Thus, dry feed systems for adding catalyst particles directly into the fluidized bed are still commonly used. Different carrier gases, e.g., nitrogen or ethylene, and different flow-rates have been employed for dry-catalyst feeding. However, the problem of hot spots forming in the reactor due to uneven catalyst mixing has not been completely solved.
In the gas-phase polymerization reactor, induced electrostatic forces may worsen the movement and dispersion of catalyst particles. To fight the resulting operational problems, such as sheeting and chunking, continuity aid (CA) has been used to reduce the electrostatic force and improve the reactor operation. However, using CA can add to the cost, reduce catalyst activity and increase complicity of reactor operation, and CA's chemical properties may cause product quality concerns for some of the food and medical applications. Reducing or eliminating the use of CA is desired, for both the dry-catalyst-feeding reactors and slurry-catalyst-feeding reactors.